Heady heights, profound depths
High and grounded on Ice Age earth
The Elbtower sets technical and planning milestones even before construction begins. Never before has the geology of a construction site in Germany been investigated so deeply.
If you want to reach for the sky in Hamburg, you have to dig deep. And this is all the more true of the Elbtower, which in just a few years will lend its elegant verve to the city. After all, Hamburg is not built on trust and confidence alone, it stands on historically grown, typically northern European soil. The ice ages of the last few hundred thousand years and millennia of human settlement history are apparent here.
It was therefore clear from the very beginning that the Elbtower would have to be firmly anchored in the ground. Such a structure can only reach for the sky if it has a deep foundation and rests on piles that safely dissipate the immense weights and forces into the subsoil.
Looking into the ground
This has planning consequences. The construction companies have to go deeper and deeper through the upper layers of the building ground. Only in the first few metres do they find traces of human occupation: fill, rubble and waste. Further down, the layers are thousands, hundreds of thousands – even millions of years old.
It took almost two years to investigate the subsoil in the area near the Elbbrücken (Elbe bridges). “We had to carry out many different subsoil explorations using different methods,” says Dr. Hatice Kaya-Sandt from the planning and consulting office GuD/BBI.
The drilling went up to 70m deep, by which the geologists and engineers were able to gather knowledge about the construction site. Even underground obstacles such as layers of rubble from the last ice ages did not frighten the engineers. “In the actual tower area, we went down to a depth of 207m. There we took samples from the ground metre by metre and examined them,” says Kaya-Sandt. Specialised soil mechanics laboratories also analysed how the rock and soil behave under the high pressure and stress of the future piles.
»In the actual tower area, we went down to a depth of 207m. There we took samples from the ground metre by metre and examined them.«
A city on the glacial gully
New findings about Hamburg’s subsoil have emerged: far down in the ground lie channels that once shaped the ice-age glaciers. No one knows exactly how deep and wide they are today, but there is clarity where the Elbe Tower is to stand. There are many alternate layers of different deposits here. The so-called Lauenburg clay – also a relic of several ice ages – stretches in various forms as a more or less thick layer from Lauenburg, 50 kilometres away, to the Elbbrücken. Some of the soil layers are only a few decimetres deep, others up to 10 metres. They are certainly not homogeneous layers as in the model, but run in waves and depressions, with many large and small transitions to other sediment layers. “A side effect of our investigations is that we have gained new insights into the deep geology in this area. The results of our investigations have therefore been gratefully received by the Geologische Landesamt Hamburg,” explains Kaya-Sandt.
But it turned out that the sediments would not provide any support for a future building and were incompatible with the loads of the Elbtower. Therefore, the tower builders have to find load-bearing layers and make sensible use of the existing subsoil in order to securely anchor the Elbtower’s pile foundations.
Test piles of record depths
After the investigations, specialised construction companies sank test piles into the ground. With a diameter of almost two metres, these cylinders of concrete and steel reach up to 111 metres deep into the ground. Never before have such long piles been driven into the ground in Germany. They are intended to prove that both the soil and the piles can safely take the immense load imposed by the future structure. “Even today, every pile drilling is exciting,” says the geotechnical engineer, “we monitor it very closely”.
With the findings from the soil investigation and test drilling, structural engineers have calculated the number and depth of the pile foundations: 63 of them are now being manufactured and sunk into the ground. They also have gigantic dimensions. With a length of 83 metres below the current site, they are taller than most of the Saturn space rockets. Their task will be to direct the Elbtower’s loads into the ground. Up to 100 tonnes of the tower’s weight will later press down on every square metre, and even over the lower floor areas the load is almost half that.